The invention pertains to stable bitumen foam and an process for preparing such bitumen foam. The invention also pertains to the use of this bitumen foam in civil engineering and the manufacturing of roof-covering.
In the sixties it was discovered that mixing bitumen and crushed stone can be improved by converting the bitumen into foam with a lower viscosity. In order to manufacture this so-called foam bitumen, Csanyi proposed in U.S. Pat. No. 2,917,395 to inject steam under a super-atmospheric pressure typically of about 3.4-6.1 atm. in a mixing chamber with bitumen, and pump out the mixture of steam and bitumen through a pipe with specific dimensions, for example at a lower pressure of 1.4-5.4 atm. This way, bitumen is produced which increases at least 10, but in general 15-20, and sometimes even 50 times in volume. However, this method seemed primitive because the quality of the foam bitumen was not consistent and it was impossible to realise a ratio of foam:bitumen of more than 4:1.
This process was improved by Mobil Oil Corporation, as described in the patent publication GB-1,325,916 from 1973. The improvement consisted of substituting steam for water, so that it was cheaper and easier to manufacture the mix. By thoroughly mixing water and bitumen at a pressure higher than the atmospheric pressure and then bringing the mixture to a lower pressure, preferably atmospheric pressure, the rapid evaporation of the water ensures consistent foaming and the degree of foaming can be regulated by the pressure difference.
Since that time, foam bitumen has always been manufactured by bringing water in contact with hot bitumen. In US 2003/0134036 an operating procedure is described that uses foamed bitumen to coat stones in an asphalt installation, using a cascade system according to the conventional “Csanyi” method.
The disadvantage of this technique is that foaming is largely determined by the water pressure, of which the vapour pressure decreases the moment the bitumen cools off to below the 100° C. and the water condenses. Thus such foam has only a limited standing time left and a very limited temperature limit. Therefore, when this foam bitumen is used as an alternative for “untreated bitumen,” for example to manufacture asphalt or for wearing courses (due to the favourable viscosity lowering effect of foamed bitumen), it should still be done at relatively high temperatures. After the application of the foam bitumen there is also only a limited possibility to include crushed stone into this before the layer slumps. Once it has slumped, a homogenous division of the crushed stone does not occur, and the asphalt or wearing course has an inferior quality. Furthermore, the water can afterwards have a negative effect on the characteristics of the asphalt. The danger of stripping, whereby the bitumen or crushed stone becomes undone, is thus not inconceivable. Water, due to its polar structure, gives a better “moistening” of polar concrete surfaces (such as applied during the preparation of asphalt) than non-polar binding agents, including bitumen.
Furthermore in GB-816,254, published in 1959, a method was described to obtain foam bitumen using a blowing medium, for example to bring carbonate salt into softened bitumen and raise the temperature higher than the decomposition temperature of the blowing medium, but whereby the temperature is indeed maintained sufficiently low as to prevent the occurring gas, for example CO2, from escaping. It typically deals with a temperature of 100-180° C., and after approximately 2 hours results in a product with a bulk density of 0.2-0.25 g/cm3. However, such a method contains chemicals that are relatively expensive to be applied on a large scale in the bitumen industry.
In U.S. Pat. No. 4,817,358 mention is made of the preparation, or preparation in batches of a thin, foamed asphalt layer by applying a high shear mixer. The asphalt temperature is between 250 and 450° F. (120 and 230° C.). As mentioned therein, stable foam can only be obtained with very thin layers, so thin that they cool off rapidly, so that the rest of the foam structure is “frozen.” In U.S. Pat. No. 4,673,614 a similar form of construction is also discussed, whereby hot asphalt is brought in. Due to the lack of further specifications it is plausible that here it deals with the same high temperatures. In practice, thin layers with these techniques mean approximately 0.5 mm, whereby they—and then even in a limited way—can only be used for a specific type of roof-covering (such as shingles). With higher thicknesses, stability is impossible. Moreover, the batch process proposed in there are not profitable, especially not with road building, where large surfaces are worked on.
The so-called air-blown bitumen also exists in the field. This designation deals mostly with the manner of preparation. Blowing bitumen is a process that is at least as old as the above-described preparations of foam bitumen. For example GB-1,237,787 describes such a process, whereby an oxygen-containing gas is blown through bitumen at a temperature high enough to dehydrogenate the bitumen under the influence of oxygen. The temperature of the bitumen is approximately 270° C., and the gas is being blown in at a minimum of 170° C. The blowing conditions are adjusted as such that a froth flow of small air bells occurs, preferably as small as possible, in order thus to optimize the contact surface between the reactants oxygen and bitumen.
It is also possible to increase the process of dehydrogenation by using catalytic agents such as described in WO-A-97/19981. In this case phosphoric acid is applied, and due to this it may already be possible to obtain a chemical change at a temperature of 190° C. and a length of time of at least 1 hour. Besides, the lowest temperature in the practical examples is still 200° C. It is also described here that a combination of bitumen and a thermoplastic rubber can be used as a base; the objective here each time is to obtain a chemical change due to polymerization.
Often even antifoam additives are added here, but in as far as foaming occurs in this process, it deals with instable foam that disappears almost instantaneously after blowing. The formulation of the problem here is completely different from the one during the manufacture of bitumen foam: due to the blowing of the bitumen (leading through of oxygen) the softening point of the bitumen is increased and penetration lowered, without this occurring at the expense of its water resistance and durability.
As for the prior art, the need for stable bitumen foam and a cost-effective, easy, and safe method to prepare bitumen foam still exists. In civil engineering for example the need also exists for a substance that is easier to process into asphalt and wearing courses without the need for high temperatures, and which allows sufficient time to divide crushed stone homogeneously.